Synthesis, Spectroscopy and Electrochemistry in Relation to DFT Computed Energies of Ferrocene- and Ruthenocene-Containing -Diketonato Iridium(III) Heteroleptic Complexes. Structure of [(2-Pyridylphenyl)2Ir(RcCOCHCOCH3]

Abstract

A series of new ferrocene- and ruthenocene-containing iridium(III) heteroleptic complexes of the type [(ppy)₂Ir(RCOCHCOR')], with ppy = 2-pyridylphenyl, R = Fc = Fe^II(η⁵-C₅H₄)(η⁵-C₅H₅) and R' = CH₃ (1) or Fc (2), as well as R = Rc = Ru^II(η⁵-C₅H₄)(η⁵-C₅H₅) and R' = CH₃ (3), Rc (4) or Fc (5) was synthesized via the reaction of appropriate metallocene-containing β-diketonato ligands with [(ppy)₂(-Cl)Ir]₂. The single crystal structure of 3 (monoclinic, P2₁/n, Z = 4) is described. Complexes 1-5 absorb light strongly in the region 280-480 nm the metallocenyl -diketonato substituents quench phosphorescence in 1-5. Cyclic and square wave voltammetric studies in CH₂Cl₂/[N(ⁿBu)₄][B(C₆F₅)₄] allowed observation of a reversible Ir^III/IV redox couple as well as well-resolved ferrocenyl (Fc) and ruthenocenyl (Rc) one-electron transfer steps in 1-5. The sequence of redox events is in the order Fc oxidation, then Ir^III oxidation and finally ruthenocene oxidation, all in one-electron transfer steps. Generation of Ir^IV quenched phosphorescence in 6, [(ppy)₂Ir(H₃CCOCHCOCH₃)]. This study made it possible to predict the Ir^III/IV formal reduction potential from Gordy scale group electronegativities, χ_R and/or Σχ_R' of -diketonato pendent side groups as well as from DFT-calculated energies of the highest occupied molecular orbital of the species involved in the Ir^III/IV oxidation at a 98 % accuracy level.

Keywords: betadiketone; crystal structure; electrochemistry; electronic spectrum; ferrocene; iridium; phosphorescence; ruthenocene; spectroelectrochemistry; substituent effects.